If a skin defect covers a wide area, the defect should be treated at an early stage. One method of regenerating skin to heal a defect involves artificial dermis tissues being regenerated in vivo by implanting a collagen sponge, which functions as a scaffold, in the body without seeding cells (this is referred to as an artificial dermis production method). The artificial dermis production method requires thin split-thickness skin grafting or transplantation of a cultured epidermis after regeneration of dermis-like tissues. In either case, regeneration of dermis components must be conducted as soon as possible.
A standard production method of artificial dermis involves obtainment of a collagen sponge by freeze-drying, and in order to control degradation and adsorption rates in vivo, a cross-linking treatment is usually conducted.
One method for promoting skin tissue regeneration involves applying a growth factor, such as a basic fibroblast growth factor (bFGF), to the skin surface. This method is somewhat effective because the applied growth factor can be adsorbed through the skin. However, the application has to be conducted every day in order to maintain the effect. Therefore, several attempts have been made to find a method of maintaining the effect with only a one-time administration by sustainedly releasing the growth factors.
For example, Kawai et al., discloses that by infusing a bFGF-impregnated gelatin microsphere into an artificial dermis, a sustained release of bFGF and construction of skin tissues in vivo can be promoted (Non-Patent Document 1). Additionally, when a cell seeded-type cultured skin is prepared, constitution of tissues is promoted by adding a bFGF-impregnated gelatin microsphere (Non-Patent Document 2).
However, such methods require impregnating bFGF in a microsphere, and further infusing the thus-prepared microsphere into an artificial dermis at a clinical site, resulting in a complicated operation.
If it is possible to impart the artificial dermis itself with the ability to release a growth factor in a sustained manner, the only procedure necessary at a clinical site is to apply a growth factor to the artificial dermis, greatly simplifying the process.
The cell growth factor most widely used in current clinical fields is basic fibroblast growth factor (bFGF). The bFGF has an isoelectric point of 9.6 and can adhere to a gelatin having an isoelectric point of, for example, 5.0, by electrical interaction. By utilizing such characteristics, it is possible to release bFGF in vivo in a sustained manner with time by using a bFGF-adsorbed microsphere, which can be prepared by impregnating a microsphere made of acid gelatin (having an isoelectric point of 5.0) with b FGB (Patent Document 1, and Non-Patent Document 3).
However, this method requires manipulation, for example, making the bFGF to be adsorbed in a particulate microsphere, and then infusing the thus-obtained bFGF-adsorbed microsphere into a substrate. This makes the method complicated and hard to use in a clinical site.
If an artificial dermis material is prepared by making the bFGF to be adsorbed in an acid gelatin, in which a bFGF is easily adsorbed, a substrate having a sustained releasing ability may be obtained. However, gelatin is inferior to collagen in vivo in terms of ease of cell infiltration, etc., and therefore it is not suitable for tissue regeneration. Accordingly, a substrate that can adsorb an adequate amount of bFGF and to which surrounding cells can easily enter is demanded.
Patent Document 2 discloses a medical device that contains gelatin and collagen as essential components, and is irradiated with ultraviolet to achieve cross-linking. The medical substrate is suitably used as a cell culturing carrier, for cultured skin, etc.    Patent Document 1: Japanese Unexamined Patent Publication No. 2003-325652    Patent Document 2: Japanese Unexamined Patent Publication No. 1999-47258    Non-patent Document 1: K. Kawai et al., Biomaterials, Vol. 21, pp. 489-499 (2000)    Non-patent Document 2: Saso et al., Japanese Journal of Burn Injuries, Vol. 29, pp. 24-30    Non-patent Document 3: Y. Tabata et al., J. Controlled release, Vol. 31, pp. 189-199 (1994)